Amorphous Ge-As-Te Thin Films Prepared by Pulsed Laser

Deposition

A Photostability Study

M. Bouška

, P. Hawlová

, V. Nazabal

1,2

, L. Beneš

and and P. Němec

Department of Graphic Arts and Photophysics, Faculty of Chemical Technology, University of Pardubice,

53210, Pardubice, Czech Republic

Institut des sciences chimuques de Rennes, UMR CNRS 6226, Equipe Verres et Céramiques,

Université de Rennes 1, 35042, Rennes, France

Joint Laboratory of Solid State Chemistry of the Institute of Macromolecular Chemistry AS CR,

v.v.i. and University of Pardubice, Studentská 84, 53210, Pardubice, Czech Republic

Keywords: Amorphous Chalcogenide, Thin Films, Ge-As-Te, Photostability.

Abstract: Pulsed laser deposition was used for the fabrication of amorphous thin films from Ge-As-Te system with the

aim to study their intrinsic photostability. Photostability of prepared layers was studied using spectroscopic

ellipsometry within as-deposited as well as relaxed layers. For irradiation, laser sources operating at three

energies (1.17, 0.92 and 0.8 eV) in band gap region of the studied materials were employed. The lowest

values of photorefraction (refractive index changes) accompanied with lowest changes of band gap values

present Ge

thin films, which are therefore considered as the layers with highest photostability,

especially in relaxed state.

1 INTRODUCTION

Amorphous chalcogenides based on S, Se and Te

elements in combination with suitable element(s)

from 14

or 15

group of periodical system

(typically Ge, As, etc.) are unique due to their

photoinduced phenomena. Irradiation with

appropriate energy and intensity may change

physico-chemical properties (refractive index, band

gap, thickness, etc.) of amorphous chalcogenide thin

films (Shimakawa et al., 1995). On the other hand,

photoinduced changes of structure and properties

limit potential applications of amorphous

chalcogenides in the field of infrared optics based on

their interesting nonlinear optical properties

(Chauvet et al., 2009).

Knowledge of photoinduced phenomena in

binary arsenic- and germanium-based amorphous

chalcogenides (Nemec et al., 2009; Sleecky et al.,

1996; Vateva, 2007) suggests that in ternary Ge-As-

Te(S, Se) materials, photodarkening (decrease of

band gap energy) and photobleaching (increase of

band gap energy), connected with positive or

negative photorefraction (refractive index changes),

could be compensated by an appropriate choice of

composition. Nevertheless, the studies leading to

optimization of intrinsic chemical composition of

amorphous chalcogenides in order to prevent

undesired photoinduced effects are rare (Yang et al.,

2008; Nemec et al., 2010; Su et al., 2013), focusing

on Ge-As-Se thin amorphous films.

Transmission window of S- and Se-based

amorphous chalcogenides in infrared is restricted by

the long-wavelength (multiphonon) absorption edge

at 11 and 15 µm, respectively (Eggleton at al.,

2011). Nevertheless, for some applications, it is

necessary to develop materials optically transparent

beyond 16 µm. This requirement might be satisfied

by use of amorphous tellurides, for example from

binary Te-X (X = Cl, Br or I), Ge-Te, As-Te or

ternary Ge-Ga-Te, Ge-Te-I, Ge-In-Te or Ge-As-Te

systems (Bureau et al., 2008; Yang and Lucas,

2009). Excellent optical transparency in the 3–20

µm spectral window as well as large refractive index

values (>3.5 at 1.55 µm) were reported for Ge-As-

Te glasses (Yang and Lucas, 2009; Hawlova et al.,

2014).

Based on interesting bulk glasses properties and

expectation of photostable thin films discovery, this

work deals with amorphous thin films from Ge-As-

103

Bouška M., Hawlová P., Nazabal V., Beneš L. and N

emec P..

Amorphous Ge-As-Te Thin Films Prepared by Pulsed Laser Deposition - A Photostability Study.

DOI: 10.5220/0005338301030107

In Proceedings of the 3rd International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2015), pages 103-107

ISBN: 978-989-758-092-5

 2015 SCITEPRESS (Science and Technology Publications, Lda.)

Te system. Specifically, the aim of this work is to

find Ge-As-Te photostable thin films in as-deposited

but preferably in relaxed (annealed) state. The term

photostability is defined here as insensitivity of the

material to light exposure in terms of constant values

of refractive index and optical band gap.

For the fabrication of Ge-As-Te amorphous thin

films, electron beam or flash evaporation was

already used (Eggleton at al., 2011; Bureau et al.,

2008). In this work, we used pulsed laser deposition

(PLD) for thin films growth. We have already shown

that PLD technique seems to be promising for

chalcogenide thin films fabrication due to its

simplicity, easy control of the deposition process,

possibility to fabricate multilayered structures and

often stoichiometric material transfer from the target

to the films (Yang and Lucas, 2009; Hawlova et al.,

2014).

Following our previous studies in Ge-As-Se

system (Nemec et al., 2010; S. H. Mohamed et al.,

2006), in this work we studied photostability/

photosensitivity for six selected compositions from

Ge-As-Te system employing three different laser

sources with photon energy close to band gap of

studied materials (amorphous chalcogenides are

generally most sensitive for exposures with band gap

light). Five compositions followed the trend of

increasing mean coordination number (MCN) from

2.4 to 2.8; last composition (Ge

MCN=2.6) was selected for a comparison with Ge-

As-Se system studied recently.

2 RESULTS

Thin films fabricated by PLD were amorphous and

homogenous according XRD patterns and SEM. The

SEM and AFM data showed smooth surface of thin

films, without cracks and corrugations (Fig. 1 and

2).

We observed only rarely sub-micrometer sized

droplets. Surface roughness (RMS) values of all thin

films determined by AFM were found to be lower

Figure 1: SEM micrograph of Ge

thin film.

Figure 2: AFM of as-deposited Ge

thin film.

than ~1.7 nm; no changes in surface roughness

values were indicated for annealed, irradiated and

post-annealing irradiated layers.

The chemical composition of fabricated layers,

as determined by SEM-EDS, is in good agreement

with the composition of used bulk targets (Hawlova

et al., 2014). The only exception is Ge

composition, where the differences between thin

film and bulk target composition are probably

caused by non-homogeneity of the starting bulk

material, which was partly crystalline. In spite of the

fact that the bulk Ge

target was

completely crystalline (this composition is located

outside the glass-forming region (Krebs and Fischer,

1970)), corresponding films were amorphous and

their chemical composition agreed well with average

composition of the used target.

Two series of Ge

100-x-y

thin films differing

in thickness (~ 270-375 and ~810-1050 nm, Table 1)

were fabricated in order to satisfy criterion of

penetration depth of the light sources used for the

exposure experiments which must be equal or larger

than the film thickness. The penetration depth

(1/abs. coef.) for 1064 nm (1.17 eV) laser light was

estimated to be 300-400 nm, light from two other

sources had penetration depth values larger than

2 µm. Because films with thickness around 300 nm

were irradiated with 1342 nm (0.92 eV) laser and

films with thickness around 1000 nm were exposed

with 1342 nm (0.92 eV) sources, above mentioned

criterion is considered as satisfied.

The thicknesses, optical band gap and refractive

indices of all thin films were determined by variable

angle spectroscopic ellipsometry (VASE) data

analysis. The applicability of used Cody-Lorentz

model for the VASE data analysis is confirmed by

low values of mean square error (MSE) of the fitting

procedure, typically MSE<6.

Table 1 show optical band gap values and

refractive indices at 1540 nm for 300 nm PLD Ge-

As-Te thin films in different states (as-deposited,

exposed, annealed, post-annealing exposed). The

data presented stand for irradiation with laser

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sources operating at 1342 nm (0.92 eV, band gap

light). Data for ~1000 nm films and other irradiation

experiments were also obtained but they are not

shown in Table 1 for clarity.

3 DISCUSSION

Performed experiments and data analysis show that

the irradiation of as-deposited Ge-As-Te layers leads

to some photodarkening effect for ~300 nm thick

films with nominal composition Ge

(partly

also Ge

); the photodarkening takes place

for 0.92 eV irradiation (ΔE

opt

up to ~0.11 eV).

Under 1.17 eV exposure, films are photostable. For

~1000 nm thick films, the impact of irradiation on

as-deposited films has following trend. Under

0.92 eV irradiation, clear photobleaching was

observed for most of the samples (ΔE

opt

up to

~0.07 eV). On the other hand, under 0.8 eV

irradiation, only weak photobleaching was detected

for Ge

and Ge

layers (ΔE

opt

to ~0.04 eV), if any. Exposure of as-deposited Ge-

As-Te films has generally only small effect on their

refractive index values at 1540 nm, if any

(Δn ≤ 0.02).

The relaxation of as-deposited films via

annealing in inert atmosphere generally results in

their bleaching (ΔE

opt

up to ~0.18 eV for

), excluding 300 nm Ge

layers which underwent darkening (ΔE

opt

up to

~0.07 eV). The bleaching of as-deposited thin films

due to annealing is connected with the decrease of

refractive index (Δn up to ~0.16 in case of

The behavior of relaxed (annealed) PLD Ge-As-

Te amorphous thin films under exposure with

different laser sources has not general trends (except

the fact that no photobleaching was identified); that

is why each composition will be commented

separately. Two compositions (Ge

and

) exhibit almost completely photostable

behavior of optical band gap in relaxed state.

Relaxed Ge

layers show photodarkening

reaching ΔE

opt

up to ~0.09 eV for 0.92 eV

irradiation. Photodarkening was observed also for

relaxed films; ΔE

opt

~0.08 eV for 0.92

eV irradiation, magnitude of photodarkening is

lower for two other irradiation sources. In case of

annealed films, photostability was

found for 1.17 eV exposures; contrary, for 0.92 and

0.80 eV irradiation, weak photodarkening is

reported. Finally, the photostability of Ge

layers in relaxed state is rather good under 1.17 and

0.92 eV irradiation; however, under 0.80 eV

exposure photodarkening with magnitude of ΔE

opt

~0.05 eV is seen. From the point of photorefraction,

four studied compositions (Ge

, Ge

and Ge

)

present almost zero photorefraction in relaxed state

under all three irradiation sources.

Taking into account all the data, lowest values of

photorefraction accompanied with lowest changes of

band gap values were identified for Ge

thin films, which are therefore considered as the

layers with highest photostability among studied

samples, especially in relaxed state. Zero

photorefraction is of high importance for some

applications of amorphous chalcogenides, such as

for laser beam propagation in nonlinear regime

(Chauvet et al., 2009). That is why pulsed laser

deposited Ge

thin films are attractive; they

are promising also due to their expected high

(non)linear refractive index.

Table 1: Optical band gap values (in eV) and refractive indices (at 1540 nm) of Ge-As-Te thin films at different stages of

the experiments (as-deposited, exposed, annealed and post-annealing exposed). Exposure experiments were performed with

0.92 eV CW laser source. Band gap values (± 0.01 eV), refractive index data (± 0.01) as well as thicknesses (± 2 nm) of two

series of fabricated films were extracted from VASE data analysis. MCN stands for the mean coordination numbers

calculated from chemical composition of the films measured by EDS. Note that data shown are for films with ~300 nm

thickness.

Nominal composition MCN Thickness (nm) Optical band gap (eV) Refractive index

as-deposited annealed as-deposited annealed

non-irrad./irrad non-irrad./irrad non-irrad./irrad non-irrad./irrad

2.50 280/1050 0.90/0.79 0.86/0.77 3.76/3.76 3.75/3.74

2.53 270/870 0.89/0.84 0.96/0.88 3.69/3.68 3.67/3.66

2.63 320/810 0.90/0.90 0.99/0.95 3.71/3.70 3.61/3.62

2.72 335/930 0.90/0.90 1.00/1.01 3.70/3.69 3.59/3.57

2.74 375/1000 0.89/0.89 1.07/1.07 3.68/3.66 3.52/3.52

2.64 330/1010 0.90/0.89 1.01/1.00 3.63/3.63 3.53/3.52

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Discovery of thin films photostability in Ge-As-Te

system, located at Ge

composition

(MCN=2.6), is coherent with our earlier work

dealing with Ge-As-Se amorphous layers, where the

photostable composition was found to be

(Nemec et al., 2010). As pointed out by

Calvez et al. (2008), photostructural changes such as

photodarkening decrease and tend to vanish in

overcoordinated glasses, i.e. when MCN= 2.6 in Ge-

As-Se system. In case of Ge-As-Te thin films

studied here, some compositions have MCN higher

than 2.6; however, they present some photoinduced

phenomena. Moreover, Ge

films having

MCN=2.6 are not completely photostable. Above

mentioned facts lead to the conclusion that MCN

does not seem to be the main decisive parameter

influencing photostability of amorphous

chalcogenides. Our conclusion is supported by the

work of Khan et al., who studied light induced

response of thermally evaporated Ge-As-Se thin

films concluding that coexisting photodarkening and

photobleaching do not show a regular trend with

respect to MCN; instead evidence that Ge: As ratio

plays important role, rather than rigidity of the

amorphous network, is provided (Khan et al., 2014).

In summary, pulsed laser deposition was

exploited for the fabrication of Ge-As-Te amorphous

thin films. Morphology of prepared films is of good

quality and their surface roughness is low.

Photostability of the layers was studied in as-

deposited as well as annealed state of the samples

under irradiation with lasers operating at 1064, 1342

and 1550 nm. Highest photostability was found for

thin films, which are therefore

promising for nonlinear applications.

4 METHODS

4.1 Samples Preparation

The targets used for PLD were bulk chalcogenide

materials with nominal composition Ge

, Ge

and Ge

. Bulk samples were

prepared by weighting high purity elements (5-6 N)

in fused silica ampoules which were evacuated for a

few hours and sealed subsequently. The sealed

ampoules were heated in a rocking furnace at 1050C

for 12 hours and then quenched in water. Finally,

glass rods were cut and polished for targets useful

for PLD.

4.2 Thin Films Fabrication

For fabrication of thin films, a KrF excimer laser

operating in UV (248 nm) was used. The laser

pulses had constant output energy of 300 ± 3 mJ per

pulse, 30 ns pulse duration and 20 Hz repetition rate.

The energy fluency was set at ~ 2.6 J.cm

-2

. Vacuum

chamber (residual pressure < 3x10

-4

Pa) was used

for the fabrication of thin films; substrates were

chemically cleaned microscope glass slides and Si

wafers. The substrates were positioned parallel to

the target at target-to-substrate distance of 5 cm.

Off-axis PLD technique with rotating target and

substrates was used to avoid deep damage of the

target and to improve the thickness homogeneity of

deposited thin films.

4.3 Photostability Experiments

Photostability was studied with as-deposited and

annealed thin films. The annealing was realized in

inert atmosphere of pure argon; annealing

temperature was 20 °C below the respective glass

transition temperature of the corresponding target

glass (Hawlova et al., 2014). The duration of

annealing was 120 min; the samples were

consequently slowly cooled down to room

temperature at 1 °C.min

-1

. The photostability

experiments were performed via exposure of thin

films by laser sources operating at 1064 nm

(1.17 eV), 1342 nm (0.92 eV) and 1550 nm

(0.80 eV) with intensity of ~160 mW.cm

-2

for

exposure time long enough (120 min) for the

saturation of the photoinduced phenomena, if any.

Laser exposures were realized in inert nitrogen

atmosphere to avoid the oxidation of the films

during the experiments.

4.4 Morphological, Compositional and

Structural Characterization

A scanning electron microscope with energy-

dispersive X-ray analyser (SEM-EDS, JSM 6400-

OXFORD Link INCA) was used for chemical

composition determination of prepared Ge-As-Te

films. SEM technique was also applied to observe

the morphology of thin films using a field-emission

gun SEM (JMS 6301F). X-ray diffraction (XRD)

technique (D8-Advance diffractometer, Bruker

AXS) was exploited to prove amorphous state of

thin layers using Bragg–Brentano θ–θ geometry with

CuKα radiation and secondary graphite

monochromator. The diffraction angles were

measured at room temperature from 5 to 65° (2θ) in

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0.02° steps with a counting time of 5 s per step.

Atomic force microscopy (AFM, Solver NEXT, NT-

MDT) was used to study topography of Ge-As-Te

thin films within typical scanned area 10 µm × 10 µm

in semicontact mode.

4.5 Optical Characterization

Optical functions (refractive indices and extinction

coefficient spectral dependences) and thicknesses of

Ge-As-Te

thin films were obtained from the analysis

of spectroscopic ellipsometry data measured using

an ellipsometer with automatic rotating analyzer

(VASE, J.A. Woollam Co., Inc.) The measurement

parameters are as follows: spectral region

300–2300 nm with 10 or 20 nm steps (depending on

thickness of the films), angles of incidence 50°, 60°

and 70°. For the analysis of VASE data we used

Cody-Lorentz model (Cody, 1984), which includes

the correct band edge function, weak Urbach

absorption tail description as well as Lorentz

oscillator function; this model is appropriate for the

description of amorphous chalcogenides optical

functions and their photo-induced changes (Nemec

et al., 2010).

ACKNOWLEDGEMENTS

The Czech Science Foundation (Project No.

13-05082S), Ministry of Education, Youth and

Sports of the Czech Republic (Project

CZ.1.07/2.3.00/30.0058) and the CNRS PICS

(Projet International de Cooperation Scientifique)

program financially supported this work.

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